The third Generation Partnership Project (3GPP) has specific functions for Evolved Packet System (EPS) networks and devices that allow them to use the traditional Circuit Switched (CS) services that are offered by Global System of Mobile Communication (GSM) and Universal Mobile Telecommunication System (UMTS) networks.
The motivation for this feature is that Circuit Switched (CS) services, such as today's voice and SMS services, are not available on the Evolved Packet System (EPS) due to the fact that Evolved Packet System (EPS) offers Packet Switched (PS) services only.
Hence functionality is needed to allow Evolved Packet System (EPS) terminals—while registered with the Evolved Packet System (EPS)—the use of GSM/UMTS Circuit Switched (CS) services in case there is no equivalent service available on the Evolved Packet System (EPS) network. For example, in the absence of a native “Voice-over-LTE” solution on the serving EPS network (or in case the terminal (or User Equipment) chooses for other reasons, such as its voice configuration, not to use the Evolved Packet System (EPS)), terminals may request the registration for Circuit Switched (CS) services via the Evolved Packet System (EPS), whereby the terminal is registered on the Circuit Switched (CS) domain of a GSM/UMTS mobile communication network (i.e. with a Mobile Switching Center (MSC) on the GSM/UMTS Circuit Switched (CS) domain) by the Evolved Packet System (EPS). Subsequently, the terminals may use the services offered by the Circuit Switched (CS) domain while they continue to be registered in the Evolved Packet System (EPS). In order to do this, the terminal tunes (from its serving cell in the Evolved Packet System (EPS)) to a GERAN (GPRS/EDGE radio access network) or UTRAN (UMTS Terrestrial radio access network) cell of a CS-enabled network when a CS service is invoked. This functionality is called Circuit Switched Fallback (CSFB) and is specified in Third Generation Partnership Project (3GPP) TS 23.272, e.g. in Version 9.5.0 of September 2010.
Circuit Switched (CS) refers to a domain of the GSM/UMTS mobile communication network whereby services are delivered via dedicated circuits.
Packet Switched (PS) refers to a network domain where services are delivered using statistical multiplexing of “packets”, whereby packets are segments of the service data. It exists in Global System of Mobile Communication (GSM), Universal Mobile Telecommunication System (UMTS) and Evolved Packet System (EPS) mobile communication network.
Global System of Mobile Communication (GSM) refers to the 2G network as defined by the Third Generation Partnership Project (3GPP). Its radio access is termed GERAN.
Universal Mobile Telecommunication System (UMTS) refers to the 3G network as defined by the Third Generation Partnership Project (3GPP). Its radio access is termed UTRAN.
Evolved Packet System (EPS) refers to the 4G network as defined by the Third Generation Partnership Project (3GPP). Its radio access is termed E-UTRAN.
General Packet Radio System (GPRS) refers to the Packet Switched (PS) part (or domain) of Global System of Mobile Communication (GSM) and Universal Mobile Telecommunication System (UMTS) mobile communication networks.
According to ETSI TR 21.905 V8.8.0, the term MME stands for Mobile Management Entity; according to TS23.401, V8.6.0 this term stands for Mobility Management Entity; hereinafter, the term Mobility Management Entity is used.
In order to use services offered by a particular network domain, terminals (or User Equipments) must be registered with (“attached” to) that domain. Terminals may be attached to a single domain of the network only, i.e. just the EPS, or, while in GSM/UMTS, just the CS domain or just the PS domain. Alternatively, terminals may be attached to two domains; i.e. in EPS, they may be simultaneously attached to the EPS and the CS domain of the GSM/UMTS network; in GSM/UMTS, they may be simultaneously attached to the CS domain and the PS domain of the GSM/UMTS network.
For each network domain that the terminal (or User Equipment) is attached to, mobility management functions are necessary to cater for the movement of the terminals. These functions comprise, inter alia, the assignment of a “location identity” in each of the domains, and a serving node within the domain that serves the location represented by the location identity, as follows:                in the CS domain of the GSM/UMTS network, a Location Area Identity (LAI) and a serving Mobile Switching Center (MSC) are assigned;        in the PS domain of the GSM/UMTS network, a Routing Area Identity (RAI) and a Serving GPRS Support Node (SGSN) are assigned;        in the EPS, a list of one or more Tracking Area Identities (TAI list) and a serving Mobility Management Entity (MME) are assigned.        
Terminals (or User Equipments) detect their LAI/RAI/TAI from information broadcast by their serving cell, whereby a GERAN or UTRAN cell may broadcast a LAI and a RAI, and an E-UTRAN cell may broadcast a TAI. Network planning must ensure that the serving cell that broadcasts a given LAI/RAI/TAI is connected with a MSC/SGSN/MME that is configured to host this Location Area/Routing Area/Tracking Area. Appropriate signaling between the terminal (or User Equipment) and the network is in place for performing updates of the assigned LAI/RAI/TAI list when the terminal moves into a serving cell that broadcasts a LAI/RAI/TAI that is different to its current assignment.
When the terminal is idle, changes of the Location Area/Routing Area/Tracking Area are notified to the network; such that:                a Location Area Update (LAU) towards the MSC is invoked when the terminal moves into a new GERAN/UTRAN serving cell that broadcasts a different LAI than the one currently assigned to the terminal;        a Routing Area Update (RAU) towards the SGSN is invoked when the terminal moves into a new GERAN/UTRAN serving cell that broadcasts a different RAI than the one currently assigned to the terminal;        a Tracking Area Update (TAU) towards the MME is invoked when the terminal moves into a new E-UTRAN serving cell that broadcasts a TAI that is not within the list of TAIs currently assigned to the terminal.Furthermore, when there is no need for the terminal to signal a change of LAI/RAI/TAI to the network (e.g. due to not moving out of its current assignment), the terminal may invoke the appropriate updating procedure based on periodicity timers provided by the network. If the terminal is registered to two domains as described above (“Alternatively, terminals may be attached to two domains . . . ”), it may use “combined” procedures whereby, as appropriate, either the RAI or the LAI or both can be updated by sending a “combined RAU/LAU” request to the SGSN when the terminal is in GSM/UMTS, or either the TAI list or the LAI or both can be updated by sending a “combined TAU/LAU” request towards the MME when the terminal is in the EPS. Combined procedures are always used in the case of terminals that are simultaneously EPS/IMSI-attached, while the use of individual or combined procedures depends on network configuration in the case of terminals that are simultaneously IMSI/GPRS-attached.        
The updating procedures described above are used by terminals that are attached to the corresponding domain(s). In order to become attached to a domain, similar procedures are available (“IMSI Attach”, “GPRS Attach”, “EPS Attach”). Simultaneous attachment to two domains (as described above) can be achieved by “Combined IMSI/GPRS Attach” or “Combined EPS/IMSI Attach”.
In contrast to LAI/RAI/TAI changes—which are always signaled when the terminal is idle and may be signaled when the terminal is active—changes of the GERAN/UTRAN/E-UTRAN serving cell are only notified to the network when the terminal is “active” (e.g. engaged in a voice call in the CS domain, or in data communication in the PS domain).
In line with the above, as a prerequisite to using the CS domain of the GSM/UMTS mobile communication network by means of Circuit Switched Fallback, terminals in EPS must register on the CS domain (“IMSI attach”) in addition to being registered with the EPS (“EPS Attach”). Since the terminal (or User Equipment) is in EPS (and hence served by an E-UTRAN cell), requests by the terminals (or User Equipments) for registration on the CS-domain are sent to the Mobility Management Entity (MME) and then relayed by the Mobility Management Entity (MME) towards the Mobile Switching Center (MSC) via the SGs interface between the Mobility Management Entity (MME) and the Mobile Switching Center (MSC). This registration on the CS domain for terminals in EPS can thereby occur as follows:                when the terminal (or User Equipment) registers with the EPS (i.e. performs an “EPS attach”): In this case, the terminal (or User Equipment) performs a “Combined Attach” as described above (“Simultaneous attachment to two domains (as described above) can be achieved by . . . ”) that results in both EPS and IMSI attachment of the terminal or User Equipment;        at a later stage after an EPS attachment: In this case, the terminal performs a “Combined Tracking Area Update” that includes the request (by the User Equipment) to become IMSI attached.        
As described above, the Tracking Area Update (TAU) is the normal location updating procedure in EPS, equivalent to, e.g., Location Area Update (LAU) in the Circuit Switched (CS) domain, or Routing Area Update (RAU) in the Packet Switched (PS) domain of GSM/UMTS mobile communication networks. In the context of Circuit Switched Fallback (CSFB), the “combined” variant of the TAU is used to indicate that it applies to both EPS and IMSI attachment of the terminal. Furthermore, as explained in the preceding paragraph, Combined TAU can be used solely to request IMSI attachment by a terminal (or User Equipment) that is already EPS attached.
While the terminal (or User Equipment) is IMSI attached, a serving Mobile Switching Center (MSC), and a Location Area (LA) “within” that Mobile Switching Center (MSC), must be assigned to the terminal (or User Equipment). This requirement applies regardless of whether:                the terminal is in GSM/UMTS and only attached to the CS domain; or        the terminal is in GSM/UMTS and attached to both the CS domain and the PS domain; or        the terminal is in the EPS and is “combined-attached” to both the EPS and the CS domain of the GSM/UMTS network.However, it is very different how this assignment is performed and maintained for IMSI-attached terminals in EPS (i.e. those that want to make use of CSFB) and IMSI-attached terminals in GSM/UMTS, as further described below.        
When the terminal (or User Equipment) is in a GSM/UMTS mobile communication network, it communicates via a GERAN or UTRAN serving cell for access to both the CS domain and the PS domain. Its Location Area and its Routing Area equate to the LAI/RAI that are broadcast in that serving cell (in other words, the Location Area (LA) and the Routing Area (RA) of the User Equipment is the Location Area (LA) and the Routing Area (RA) of which the serving GERAN/UTRAN cell is a member), and the Mobile Switching Center (MSC)/SGSN are determined based on network topology (i.e. a Mobile Switching Center (MSC)/SGSN is used that hosts the terminal's serving cell for the CS domain and the PS domain, respectively).
When the terminal (or User Equipment) is in the Evolved Packet System (EPS), it communicates via an E-UTRAN serving cell for access to the PS domain, and needs to switch to a GERAN/UTRAN cell when CSFB is invoked. This means that there is no serving GERAN/UTRAN cell for such a terminal in EPS and hence no LAI can be derived from the GERAN/UTRAN cells. Therefore, when the terminal signals to the MME that it wishes to become IMSI attached, the assignment of the Location Area in the Circuit Switched (CS) network and the selection of the appropriate Mobile Switching Center (MSC) (i.e. a Mobile Switching Center (MSC) that hosts this Location Area), is performed by the Mobility Management Entity (MME) based on local configuration, taking into account, e.g., the terminal's current Tracking Area in the Evolved Packet System (EPS) and its current serving E-UTRAN cell. In order to cater for the movement of the terminal, a new Location Area (LA) and if applicable—i.e. in case that the new Location Area (LA) belongs to another Mobile Switching Center (MSC)—a new Mobile Switching Center (MSC) may be selected by the Mobility Management Entity (MME), and assigned to the terminal, when the terminal signals a combined Tracking Area/Location Area Update towards the MME.
The triggers for sending a combined TAU/LAU are specified in 3GPP TS 24.301 and are independent of the LAI of any overlapping GSM/UMTS cell, which could serve as a potential CSFB target cell. While the terminal moves within the EPS (i.e. uses a E-UTRAN serving cell), a (combined) TAU(/LAU) is typically signaled when the UE moves into a new E-UTRAN serving cell whose TAI is not in the terminal's current TAI list, or—in the absence of any such mobility-caused TAU—when a periodic TAU timer provided by the network expires. Consequently, assignment of a LAI and serving MSC to the terminal does not take into account the (change of) LAI and hosting MSC of potential CSFB target cells in GERAN/UTRAN; rather, it is solely based on EPS network location information and associated updating triggers.
In the context of 3GPP Circuit Switched Fallback (CSFB), terminals (or User Equipments) may attach to the Circuit Switched (CS) domain for SMS only, or for all services offered by the Circuit Switched (CS) domain of the GSM/UMTS network:                Circuit Switched (CS) domain registration for SMS only (“SMS over SGs”): In this case, the terminal may send and receive SMS messages while it stays on the E-UTRAN radio access, whereby the messages will be transferred via the Mobile Switching Center (MSC); i.e. mobile originating messages will be sent by the terminal (or User Equipment) via E-UTRAN to the Mobility Management Entity (MME), which will pass them on to the Mobile Switching Center (MSC) via SGs, and the Mobile Switching Center (MSC) then passes them further on towards the Short Message Service Center (SMSC) as in the traditional GSM/UMTS CS-based SMS service. The inverted sequence (i.e. Short Message Service Center (SMSC)→Mobile Switching Center (MSC)→Mobility Management Entity (MME)→E-UTRAN→Terminal/User Equipment) takes place for mobile terminated SMS. In this mode of operation, the terminal (or User Equipment) always stays in E-UTRAN, i.e. it does not perform a fallback to a GERAN/UTRAN cell. Consequently, the choice of Mobile Switching Center (MSC) and Location Area (LA) is not relevant for the execution of the service (it is only relevant when the terminal (or User Equipment) moves between E-UTRAN coverage and GERAN/UTRAN coverage). This mode of operation is not relevant in the context of the present invention.        Circuit Switched (CS) domain registration for all Circuit Switched (CS) services: In this case the terminal will use SMS as described above, but for all other Circuit Switched (CS) services, the terminal or User Equipment performs a “fallback” to a radio access of the CS domain, i.e. a GERAN or an UTRAN cell. When a fallback is necessary, i.e. a Circuit Switched (CS) service other than SMS is invoked, a suitable GERAN or UTRAN target cell for the fallback is chosen by the Evolved Packet System (EPS) network, and the terminal is then commanded to tune to that (target) cell for the execution of the Circuit Switched (CS) service. In this mode of operation, it is highly beneficial for the service execution that the target cell of the fallback is within the Location Area (LA) that is currently assigned to the User Equipment, and that this actual Location Area (LA) (i.e. the one of the target cell)—and hence the target cell—is served by the Mobile Switching Center (MSC) currently assigned to the User Equipment.        
The Circuit Switched Fallback (CSFB) operation as specified according to the Third Generation Partnership Project (3GPP) has a number of drawbacks.
Lack of Location Area/serving Mobile Switching Center (MSC) awareness when assigning LAC and MSC to the terminal:
As described, the Mobility Management Entity (MME) assigns the Location Area (LA) and the Mobile Switching Center (MSC) to the Circuit Switched Fallback (CSFB) terminal (or User Equipment). The selection of the Location Area Code (LAC) and the Mobile Switching Center (MSC) can only occur when the terminal becomes registered on the Circuit Switched (CS) domain via the MME, or when it performs a (combined) Tracking Area Update (TAU) (the triggers for TAU are specified in 3GPP standards). However, no information of the topology of the overlapping GSM/UMTS mobile communication network—especially the LAI and the hosting MSC of neighboring GERAN/UTRAN cells that would be potential CSFB target cells—is available when the selection is made. Rather, the choice of the Location Area Code (LAC) and Mobile Switching Center (MSC) in the GSM/UMTS Circuit Switched (CS) domain has to be based on EPS/E-UTRAN related location information of the terminal, i.e. Tracking Area identity and E-UTRAN serving cell identity.Both the identity of the Tracking Area (TA) signaled by the terminal (or User Equipment), and the E-UTRAN cell in which the terminal communicates when it sends the Combined Attach or Combined TAU, may be taken into account by the Mobility Management Entity (MME) to identify the best-suited LAC, and choose a corresponding MSC. In this context, “best-suited” implies that there is a maximum likelihood that, when the User Equipment falls back to a GERAN or UTRAN cell in the Circuit Switched (CS) network, the target cell of the fallback is a member of the currently assigned Location Area, and hence hosted by the currently assigned Mobile Switching Center (MSC). This likelihood, however, depends on the degree of overlap between the EPS Tracking Areas and the GSM/UMTS Location Areas, which in turn depends on the willingness and ability of the Network Operator to achieve this overlap. For example:                Optimizing the network for this overlap by network planning deprives Operators of the highly desirable independent optimization of both the EPS and the GSM and UTRAN networks. It also constitutes a problem due to the fact that a terminal may be assigned only one Location Area (LA) in GSM/UMTS, but one or more Tracking Areas in EPS;        The level of overlap that can be achieved also depends on the different radio frequencies being used in GSM, UMTS and EPS, as the propagation characteristics of the radio signals are very different between e.g. GSM900 and EPS2600;        If the configuration/topology of at least one of the two network layers—EPS and/or the GSM/UMTS Circuit Switched (CS) network—changes, the level of overlap between them changes as well. This represents a constant operational burden on the Operator. In case, e.g., a new Evolved Packet System (EPS) network is still in the roll-out/expansion process, it is very difficult and cumbersome to manage the overlap between the Location Areas (LAs) and the Tracking Areas (TAs).In summary, the level of overlap is difficult to maintain and will always be a compromise between various factors.In case that there is a mismatch of the Location Area (LA) when the User Equipment performs a fallback, a LAU procedure has to be performed prior to setting up the Circuit Switched (CS) service, e.g. a voice call, adding significant delay to the call setup time. In the worst case, even the Mobile Switching Center (MSC) hosting the target cell of the fallback is not the one that is currently assigned to the terminal—additional delay in setting up the call incurs for both mobile originated and mobile terminated calls. In the latter case, the call attempt will even fail, and additional functionality in the network (“roaming retry”) is needed to recover from that failure by performing a second call attempt to the “correct” Mobile Switching Center (MSC), adding even more delay to the call setup time.The effect of this problem (delay in call setup) doubles in principle when both calling party and called party employ CSFB.        
Insufficient precision in relation to updating the LAI and MSC assignment of the terminal:
As described above, LAI and MSC reassignment for a CSFB terminal can only be performed when a TAU occurs. However, the TAU can only convey location information relative to the E-UTRAN cell and its TAI, but not information relevant to the LAI and hosting MSC of potential CSFB target cells in GERAN/UTRAN. In addition, mobility-triggered TAU only occurs when the terminal enters a cell outside of its TAI list, whereas in GSM/UMTS, an update of the LAI would have to occur when the terminal moves into a cell with a different, single, LAI than the one that is currently assigned. It is therefore very likely that the LAI of the surrounding GERAN/UTRAN cell changes while the terminal stays within a set of E-UTRAN cells coherent to its TAI list. If this happens, a mismatch of the LAI, and possibly the hosting MSC, occurs upon CSFB execution.
Lack of knowledge of current GERAN/UTRAN radio environment when performing a fallback:
When a fallback is needed, the terminal is sent to a GERAN or UTRAN cell in the CS domain in order to execute the CS service. As described above, a suitable target cell is selected by the EPS network, where in this context the term “suitable” implies that the target cell provides sufficient coverage for the service delivery to the UE. If this is not the case, a handover (to another GERAN/UTRAN cell) may be needed shortly after the fallback, or in the worst case the call setup will fail if the chosen target cell does not provide decent coverage for the UE when the fallback has been performed.To avoid this from happening, the 3GPP standards foresee that, when a fallback is triggered, the terminal can be commanded by the network to perform GERAN/UTRAN neighbor cell measurements in order to identify potential target GERAN/UTRAN cells for the fallback. Once these measurements are completed, the network picks the preferred target cell based on the evaluation of the measurement results. While this assures that the chosen target cell will provide good coverage for the UE after fallback, it is also obvious that these measurements increase the overall call setup time in Circuit Switched Fallback (CSFB).In order to avoid the additional delay incurred by the above measurements, it is also possible to perform a so-called “blind handover”. In this case, the CSFB target cell in GERAN or UTRAN is chosen based on the terminal's current E-UTRAN cell, i.e. the E-UTRAN cell from which the CSFB is triggered. This relies on the assumption that for every serving E-UTRAN cell in the EPS network, at least one suitable target cell in GERAN or UTRAN is known. However, this is very unreliable as the exact location of the UE within the E-UTRAN cell's perimeter is not known, and also due to the different propagation characteristics of the E-UTRAN cells (e.g. 2600 MHz) compared with GERAN cells (e.g. 900 MHz). In conclusion, blind handover may only be suitable—in terms of acceptable fallback failure rates—for collocated cells in similar frequency bands (e.g. a E-UTRAN cell in the digital dividend band collocated with the GERAN cell in the 900 MHZ band, whereby the collocated GERAN cell is chosen as the CSFB target cell for every fallback that is triggered in the said E-UTRAN cell).
Lack of Location Area/serving Mobile Switching Center (MSC) awareness of the target cell when the fallback occurs:
As described above, a target cell for the fallback is chosen by the network from the list of suitable cells that is obtained via configuration, or by the measurements performed by the terminal as commanded by the network. However, the choice of target cell does not take into account whether the target cell is in the same location area as the one that is currently assigned to the terminal, which increases the probability of a LAI mismatch (and possibly a MSC mismatch) even if cells in the same LA, or at least within another LA that is hosted by the currently assigned MSC, are available for the target cell choice.